"In case the rewritten DN is an LDAP URI, the operation is initiated
towards the host[:port] indicated in the uri, if it does not refer to
the local server"

That sentence belongs to a "white paper" that eventually evolved into a
man page; unfortunately, items on a wish list seem to have slipped in.

That thought crossed my mind... :)

That feature has never existed, although it could be useful.

Oh bummer...

In principle, you could use back-dnssrv to have requests turned into
referrals based on DNS SRV entries (draft-ietf-ldapext-locate); the
referrals could be automatically chased by an instance of
slapo-chain(5). This requires your DNS to expose the DNS SRV for ldap
regarding those servers that service each naming context you want to be
mapped. Otherwise, I think the feature you need could be added in
slapd-meta(5) somehow (and might be of general use). You could submit a
feature request via the ITS <http://www.openldap.org/its/> (best
accompanied by a patch :)